The present invention relates to an improvement in an image forming apparatus for forming a character or a figure with microminiature dots and, more particularly, to an image forming apparatus for forming an image on a photosensitive unit by at least two shutter arrays having different number of dots (dot density) formed per unit length to form microminiature dots to transfer the image on a recording medium.
Recently, a liquid crystal unit is used for an office automation equipment such as a display unit or an electrophotographic printer of a terminal unit for a computer. The liquid crystal unit used for the electrophotographic printer has in general the following structure. A pair of substrates are opposed through a liquid crystal substance. A plurality of signal electrodes are provided on the inner surface of one of the substrates. A plurality of common electrodes are formed on the inner surface of the other of the substrates. Thus, a plurality of microshutters for turning ON and OFF a light are provided. The microshutter is composed of a part cf the portion that the signal electrode and the common electrodes are opposed, and a liquid crystal interposed between the electrodes. The liquid crystal substance is sealed by a sealing member. The sealing member is formed along the outer shells of the pair of substrates. A plurality of microshutters are arranged to form a shutter array.
The liquid crystal unit of the above-mentioned construction is sometimes used as a photowriting in an electrophotographic printer. In this case, a liquid crystal unit is interposed between a light source and a photosensitive unit in the printer, and a liquid crystal drive circuit applies a signal to the signal electrodes and common electrodes of the liquid crystal unit. The microshutter for forming the shutter array of the liquid crystal unit is selectively driven by the signal to "open" (passing the light) or "close" (interrupting the light) to control the transmission of the light from the light source. Thus, a microminiature dotted image is focused on the photosensitive unit. A plurality of the dotted images are combined to form an electrostatic latent image representing a character image on the photosensitive unit. The latent image formed on the photosensitive unit is converted to a visible image, which is transferred to a sheet and printed.
Of a conventional electrophotographic printer using such a liquid crystal unit, the case using a liquid crystal of twisted nematic mode (TN mode) for the liquid crystal unit is disclosed, for example, in U.S. Pat. No. 4,386,836. This liquid crystal unit has the following construction. A nematic liquid crystal is sealed between a pair of opposed substrates. The liquid crystal molecules of the liquid crystal are twisted and oriented until the molecules arrive from one substrate at the other. A pair of polarizing plates in which the polarizing axes are perpendicularly crossed to one another are disposed outside the pair of substrates. A microshutter formed at part of the portion that a plurality of signal electrodes and a plurality of common electrodes are respectively opposed is formed in an equal size of a region for controlling the passage of the light. The microshutters are arranged in one or more rows to form a shutter array. The liquid crystal unit is driven by two frequency addressing scheme to accelerate the responding velocity. In other words, when the voltage of low frequency f.sub.L is applied to the signal electrodes and the common electrodes, liquid crystal molecules cross at their long axes to the electrode surface. Thus, the microshutter interrupts the light to become OFF. When the voltage of high frequency f.sub.H is applied between the electrodes, the molecular long axes of the liquid crystal are parallel to the electrode surface, twisted and oriented. Thus, the microshuter passes the light to become ON.
The case that a liquid crystal unit of Guest-Host effect mode is used for an electrophotographic printer is disclosed in U.S. Ser. No. 630,957 filed by the same assignee as the present invention. In other words, this liquid crystal unit has the following construction. A nematic liquid crystal added with dichromatic dye is sealed between a pair of opposed substrates. The liquid crystal molecules of the liquid crystal are homogeneously oriented. At least one polarizing plate is disposed outside the pair of substrates. A microshutter formed at a part of the portion that a plurality of signal electrodes and a plurality of common electrodes are opposed is formed in equal size in regions for controlling the passage of the light. The microshutters are arranged in one or more rows to form one shutter array. This liquid crystal unit is driven by two frequency addressing scheme so as to accelerate the responding velocity. In this case, when the voltage of low frequency f.sub.L is applied between the signal electrodes and the common electrodes, the molecule axes of the liquid crystal cross perpendicularly to the electrodes. The molecule axes of dichromatic dye which behave together with the liquid crystal molecules also cross perpendicularly to the electrodes. Therefore, the dye does not absorb the light, and the microshutter passes the light to become ON. When the voltage of high frequency f.sub.H is applied between the electrodes, the molecule axes of the liquid crystal become parallel to the electrode surface. Thus, the molecule axes of the dye become parallel to the electrode surface. Then, the dye absorbs the light of the specific wavelength band for the dye. Therefore, the microshutter interrupts the light of the specific wavelength band to become OFF.
The above-mentioned conventional liquid crystal unit has a sole shutter array formed by arranging the microshutters having equal size of regions (openings) for controlling the light in a predetermined number (arraying density) per unit length. Thus, the conventional electrophotographic printer using the liquid crystal forms an image having a dot density corresponding to the arraying density of the shutters in one type of shutter array provided in the liquid crystal unit and dot density of 1/ integer number of the corresponding dot density, and merely prints the image.
The arraying density of dots for forming a character or a figure in general employs 240 dots per inch (DPI), 300 dots per inch (DPI) or 480 dots per inch (DPI). The dot arraying density is determined in response to the sort or the size (the number of points) of a type unit of the case for printing a character. For example, kanji in Japan is frequently printed in the arraying density of 240 DPI. and alphabetic character frequently printed in the U.S.A. or Europe in the arraying density of 300 DPI or 480 DPI.
However, the conventional electrophotographic printer having a liquid crystal unit has a sole shutter in which the magnitude of a region for controlling the light passing state is equal in its liquid crystal unit and a plurality of microshutters of equal arraying density per inch are arranged.
Therefore, the above-mentioned conventional electrophotographic printer cannot print by using type units of arbitrary sort, nor print with type units of arbitrary size. Further, the electrophotographic printer usirg the conventional liquid crystal unit cannot be used except the case of receiving an image signal according to the specific dot arraying density specified by the arraying density of the microshutters, and is limited in utility. If the electrophotographic printer is used for the utility of receiving an image signal according to different dot arraying density, it requires an expensive converter for converting the image signal into a signal according to the specific dot density of the printer.